Calculating Bmax and primary turns etc...

[guitar_joe]

I'm quite confused about this...

Is it a frequency dependant characteristic of a core?
It seems really difficult to find but i looked it up on a data sheet for etd39 and it was rather confusing. I saw someone quote bmax for an etd39 as 100mT on this forum so i thought hey why not, i calculated 76. turns for primary with a input voltage of 380 dc and a switching freq of 100k. does this sound right?

I entered the same on this nifty site
http://schmidt-walter.fbe.fh-darmstadt.de/smps_e/smps_e.html

it told me to use 85 turns for the primary, also it seems to think bmax is 130mT......

Will either of these calculations be sufficient to keep the transformer out of saturation??

 

[richie00boy]

Sounds OK what you have done. That site is handy.

100kHz I think is a bit high switching frequency though.

 

[guitar_joe]

perhaps you are right, do you think 50khz would be a decent frequency.

This smps is basically my little project for this year for powering valve amplifiers.

 

[darw82]

Is it a frequency dependant characteristic of a core?

Yes, for example 3C90 material has maximum operating freq 100Khz, while 3F3 material have max. operating freq 300khz

I usually use 3E25 material for switching above 100khz
For 100khz or lower, 3C90 is the best. You can get them in Farnell.

Also you may want to read this application note, there is a lot of information about transformer.
http://www.mag-inc.com/Design Application Notes.pdf

I entered the same on this nifty site
http://schmidt-walter.fbe.fh-darmst...s_e/smps_e.html

That website will suggest you to design useless oversized transformer.

Anyway, for isolated PSU, I think 100khz switching freq is okay. I usually design an isolated power supply with sw. freq. around 100-200khz to minimize output filter.

 

[AndrewT]

Hi Guitar,
don't tell the tube guys about your smps proposal.